The invention relates to a process for the manufacture of methanol from carbon monoxide and hydrogen containing mixtures and more particularly to the manufacture of methanol in the presence of a catalyst system, obtainable by combination of at least
an alcoholate, derived from an alkali metal or alkaline earth metal, and PA1 a salt containing a cation of a metal of Group VIII of the Periodic Table of the Elements and more preferably of nickel, palladium or cobalt. PA1 Embodiments of such a process are in principle known from the U.S. Pat. Nos. 4,613,623, 4,614,749, 4,619,946 and 4,623,634. PA1 (a) a salt containing a cation of a metal of group VIII of the Periodic Table of the Elements, PA1 (b) an alcoholate from an alkaline metal or alkaline earth metal, in a single pass through reactor, by converting a carbon monoxide-hydrogen gas mixture, which has been obtained by steam treatment of natural gas and scrubbing carbon dioxide from the thus produced gas mixture and which has a hydrogen-carbon monoxide molar ratio in the range of from 2.8-4.5, using the off-gas from the reaction, after recovery of methanol, as fuel to the reformer furnace.
In this literature there is commonly disclosed a low temperature methanol preparation process, preferably using a catalyst system comprising sodium hydride-sodium alkanolate, containing 1-6 carbon atoms and nickel(II) acetate, optionally in combination with a metal carbonyl of a group VI metal and more particularly Mo, Cr or W metal, while as most preferred catalyst precursor is proposed a combination of nickel(II) acetate, sodium tert-amyl alcoholate and/or tert-amyl alcohol and sodium hydride.
In these known processes a stoichiometric gas ratio of the carbon monoxide hydrogen starting mixtures is preferred.
Although these catalyst systems on laboratory scale might enable the preparation of methanol in improved yields and under more economical operational conditions (lower temperatures and pressures) as compared to the currently used industrial scale methanol manufacturing processes (operating at high pressures and temperatures, which involve high, economically unattractive equipment and operational costs) they were not applied up to now to industrial scale processes for methanol manufacture.
A problem to be circumvented during evaluation of the laboratory scale processes according to the before-mentioned patent specifications, to an industrial process showing a desired high single pass through conversion is the use of a synthesis gas of stoichiometric hydrogen/carbon monoxide ratio. Such a gas mixture having a ratio in the range of from 1.5-2.5, is usually obtained by partial oxidation of natural gas with pure oxygen. The use of pure oxygen, however, implies the need for an air separation plant, which is costly to build and to operate. A synthesis gas of stoichiometric hydrogen/carbon monoxide ratio, but in a form diluted with nitrogen, can be obtained in an economically more attractive way by partial oxidation of natural gas with air.
However, when using such a nitrogen-diluted gas of stoichiometric ratio of hydrogen and carbon monoxide, in a process for methanol manufacture at economically attractive conversion levels, an off-gas is formed which is difficult to burn due to a too low caloric value, and at less attractive conversion levels a substantially lower yield of methanol is obtained, and substantial proportion of the energy incorporated in the original feedstock is contained in the off-gas in the form of unconverted carbon monoxide and hydrogen. Although this off-gas can now be burned, a good outlet for the energy incorporated in the off-gas will in most cases not be available.
More particularly such an outlet could not be found for a stand-alone process plant for methanol production in a remote location.
On the other hand it is known from e.g. the published German application DE-A-3244302 to produce methanol from a hydrogen and carbon monoxide containing synthesis gas. This synthesis gas produced by steam reforming of natural gas and the like consisting of light hydrocarbons, contains hydrogen in excess of the stoichiometric ratio for methanol formation. Part of the non converted synthesis gas which is recycled to the reactor, is drawn off as off-gas. From this off-gas at least a carbon monoxide containing flow is separated and recycled in the synthesis gas.
It will be appreciated, that the production of gas flows, having the desired hydrogen/carbon monoxide ratio by increasing the carbon monoxide content of the gas, will inevitably be accompanied by significantly cost increasing operations e.g. by the use of membranes, because alternative conversion of hydrogen with inexpensive carbon dioxide, if available, into methanol was surprisingly found not to be applicable with the before described specific catalyst systems. Therefore a person skilled in the art who has to search for an improved, economically attractive industrial process for methanol manufacture, would certainly not be inclined to try primarily to evaluate further the concept of a combination of the before-mentioned low temperature methanol production processes according to U.S. Pat. Nos. 4,613,623; 4,614,749; 4,619,946 and 4,623,634 and steam reforming of light hydrocarbons.
This generally appreciated conception is also actually confirmed by the disclosure in "Low Temperature Methanol Process", T. E. O'Hare et al, Am. Chem. Soc. 21st State of the Art Symposium, Marco Island, Fla., June 1986.
From this publication a clear preference of people skilled in the art can be derived for combinations of low temperature methanol producing processes, using the before described catalyst systems in a liquid form and characterized by much smaller or even completely eliminated recycling of unreacted gas, and partial combustion of natural gas with air, avoiding the use of expensive air separation equipment for the preparation of pure oxygen or oxygen enriched gases, which were necessary to prevent building up of inert nitrogen in the relatively large recycle stream, applied for prior art methanol processes.
Due to the great demand for cheap methanol in very large amounts for application as fuel and as starting material for further chemical syntheses, there is still a strongly urgent need for an economically attractive industrial bulk manufacturing process of methanol, starting from cheap starting materials and operating at attractive economical and environmental conditions, i.e. using rather simple equipment and giving a significant reduction of the methanol cost price.
An object of the present invention is therefore the development of such an industrial process for methanol manufacture.
As a result of extensive research and development a process was found, which meets the hereinbefore mentioned requirements.